Thought I'd give this a shot here.
My dads Huawei P20 has all of the sudden started sounding an alert every morning at 7am.
I factory reset his phone twice now.
The first time I loaded back up everything including his previous apps.
The second time I only reloaded his contacts off Google and went into play to reload WeChat WhatsApp and Google Pinyin....I also did not reconnect to wifi as not to automatically download the latest android updates or anything like that.
Same thing... alert at 7am for 2 minutes, nothing shows on screen nothing, my dad also said he would power off the phone and the thing would still go off.
Kind if weird because if you set an alarm and power off the phone, the phone turns itself back on when it's time for that set alarm to come on.